This invention relates generally to the fields of lithography and semiconductor fabrication. More particularly, the invention relates to the use of certain novel polymer blends that are especially useful in photoresist compositions, including ultraviolet, electron-beam, and x-ray photoresists.
There is an ongoing need in the electronics industry for increasingly higher circuit densities in microelectronic devices made using lithographic techniques. One method of increasing the number of components per integrated circuit (xe2x80x9cchipxe2x80x9d) is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This decrease in feature size has been accomplished over the past twenty years by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 nm) to the deep ultraviolet (DUV;  less than 248 nm). Development of commercial lithographic processes using ultra-deep ultraviolet radiation, particularly at 193 nm or 157 nm, is now of increasing interest. See, for example, Allen et al. (1995), xe2x80x9cResolution and Etch Resistance of a Family of 193 nm Positive Resists,xe2x80x9d J. Photopolym. Sci. and Tech. 8(4):623-636, and Abe et al. (1995), xe2x80x9cStudy of ArF Resist Material in Terms of Transparency and Dry Etch Resistance,xe2x80x9d J. Photopolym. Sci. and Tech. 8(4):637-642.
Attempts have been made to develop 157 nm resists, for example by using heavily fluorinated materials such as polytetrafluoroethylene (e.g., Teflon AF(copyright); see Endert et al. (1999), Proc. SPIE-Int. Soc. Opt. Eng, 3618:413-17) or hydridosilsesquioxanes (see U.S. Pat. No. 6,087,064 to Lin et al.). These materials do not, however, have reactivity or solubility characteristics suitable for lithographic manufacturing processes. The challenge in producing chemically amplified resists for 157 nm lithography is to achieve suitable transparency at this wavelength in polymers that have acid-labile functionalities, and that can be developed with industry-standard developers in either exposed or unexposed areas depending on whether the resist is positive or negative.
Homo- and copolymers of methyl xcex1-trifluoromethylacrylate (MTFMA) and its derivatives have been found to be surprisingly transparent at 157 nm, with an optical density (OD) of less than 3/xcexcm, whereas poly(methyl methacrylate) (PMMA) is highly absorbing (OD=6/xcexcm) (see, for example, Ito et al. (2001), xe2x80x9cPolymer Design for 157 nm Chemically Amplified Resists,xe2x80x9d Proc. SPIE 4345: 273-284; Ito et al. (2001) xe2x80x9cNovel Fluoropolymers for Use in 157 nm Lithography,xe2x80x9d J. Photopolym. Sci. Technol. 14:583-593, and Chiba et al. (2000), xe2x80x9c157 nm Resist Materials: a Progress Report,xe2x80x9d J. Photopolym. Sci. Technol 13:657-664.)
Unfortunately, MTFMA and its derivatives do not readily undergo radical homopolymerization, and polymers can be made only by anionic polymerization (see Ito et al. (1981), xe2x80x9cMethyl xcex1-Trifluoroacrylate, an E-Beam and UV Resist,xe2x80x9d IBM Technical Disclosure Bulletin 24(2): 991). Although MTFMA-methacrylate copolymers using anionic polymerization are highly useful as 157 nm resist polymers, it is still desirable to identify comonomers that polymerize with xcex1-trifluoromethylacrylic monomers by radical initiation. Radical polymerization is easy to run and economical, and is a preferred process for preparation of resist polymers.
Several polymers have now been identified as suitable components of 157 nm resist polymers. For example, copolymers of t-butyl-xcex1-trifluoromethylacrylate (TBTFMA) and bicyclo[2.2.1]hept-5-ene-2-(1,1,1-trifluoro-2-trifluoromethylpropan-2-ol) (NBHFA) have been shown to be particularly suitable. See, for example, Ito et al. (2001) Proc. SPIE 4345: 273-284, supra; Ito et al. (2001) J. Photopolym. Sci. Technol. 14:583-593, supra, and Chiba et al. (2000), supra. As norbornene copolymers based on NBHFA are made by metal-mediated addition polymerization, copolymers that can be readily prepared via a conventional radical mechanism have also been sought. Unfortunately, it is difficult to incorporate more than 50 mol % NBHFA in the copolymer, and the OD of P(TBTFMA-NBHFA) ranges from 3.2 to 2.7/xcexcm, depending on the molecular weight (see the aforementioned references).
Although the lowest OD achieved with P(TBTFMA-NBHFA) may be adequate for some purposes, it is still desirable to increase the transparency of the polymer for 157 nm applications. Furthermore, resist polymers must possess many properties in addition to good transparency at the exposure wavelength. In fact, the ability of the resist polymer to be developed in aqueous base is critically important in generating high-resolution images. Unfortunately, however, resists based on copolymers such as poly(TBTFMA-co-NBHFA) do not develop well in aqueous base due to their low hydrophilicity.
While two different polymers do not, in general, mix homogeneously, it has now been discovered and is herein disclosed that certain copolymersxe2x80x94such as the TBTFMA-NBHFA copolymer, copolymers of (4-(1-hydroxy-2,2,2-trifluoro-1-trifluoromethyl)ethylstyrene) (STHFA) with t-butyl methacrylate (TBMA) (PF-ESCAP) and with TBTFMA (PF2-ESCAP), and certain TBTFMA-vinyl ether copolymersxe2x80x94are capable of blending substantially homogeneously with other polymers, such as a homopolymer of NBHFA (PNBHFA). The blending of a lipophilic copolymer with a transparent, hydrophilic polymer improves aqueous base development and increases transparency, to allow for the generation of high-resolution images.
Accordingly, it is a primary object of the invention to address the above-described need in the art by providing a substantially homogeneous polymer blend that is suitable for use in lithographic photoresist compositions.
It is another object of the invention to provide a lithographic photoresist composition containing a substantially homogeneous polymer blend.
It is still another object of the invention to provide a method for generating a resist image on a substrate using a photoresist composition as described herein.
It is a further object of the invention to provide a method for forming a patterned structure on a substrate by transferring the aforementioned resist image to the underlying substrate material, e.g., by etching.
It is still a further object of the invention to provide a method of preparing a copolymer suitable for use in lithographic photoresist compositions.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one aspect of the invention a substantially homogeneous polymer blend comprising a first polymer and second polymer is provided. Alternatively, a third polymer may be included in the blend. The first polymer is comprised of monomer units having the structure of formula (I): 
wherein R1 is C1-12 alkyl or C1-12 fluoroalkyl, R2 is C1-12 fluoroalkyl, and L is C1-6 alkylene or C1-6 fluoroalkylene. In preferred embodiments, the second polymer is a copolymer comprising:
a first monomer unit having the structure of formula (II): 
wherein R3 is H, F, CN, CH3 or C1-6 fluoroalkyl (with fluorinated methyl groups, i.e., CF2H, CFH2, and CF3, being preferred C1-6 fluoroalkyl substituents), R4a and R4b are H or F, and R5 is CN or COOR, wherein R is selected from the group consisting of H, C1-12 alkyl, and C1-12 fluoroalkyl, or is selected so as to render R5 acid-cleavable; and a second monomer unit selected from the group consisting of: 
wherein in formulae (I) and (IV), R1, R2, and L are as defined previously, and in formula (III), R6 is H, C1-12 alkyl, C1-12 fluoroalkyl, C3-15 alicyclic, or fluorinated C3-15 alicyclic, R7 is C1-12 alkyl C1-12 alkyl substituted with 1-12 fluorine atoms and 0-2 hydroxyl groups, C3-15 alicyclic, or fluorinated C3-15 alicyclic, or R6 and R7 together form a five-, six-, or seven-membered ring, R8 is H, C1-12 alkyl, or C1-12 fluoroalkyl, and R9 is H, C1-12 alkyl, or C1-12 fluoroalkyl, or R7 and R9 together represent xe2x80x94Xxe2x80x94(CR10 R11)n xe2x80x94, in which case R6 and R8 are H, X is O or CH2, n is 1 or 2, and R10 and R11 are H, C1-12 alkyl, or C1-12 fluoroalkyl, or together form an oxo moiety (xe2x95x90O). When such an oxo moiety is present, n is 1. As an alternative to the aforementioned definitions of R6 and R8, the two substituents together form a five-, six- or seven-membered ring. Further, any carbon atoms indicated in the structures as unsubstituted may in fact be substituted with one or more inert, nonhydrogen substituents such as, for the alicyclic groups (see formula I), F, or C1-6 fluoroalkyl (preferably fluorinated methyl, ie., CFH2, CHF2 or CF3), and for the benzene ring (see formula IV), F, C1-12 alkyl, C1-12 alkoxy, C2-12 alkenyl, C2-12 alkenyloxy, C1-12 fluoroalkyl, C1-12 fluoralkoxy, and C2-12 fluoroalkenyl, wherein any of the aformentioned substituents, with the exception of F, may be further substituted with additional moieties, e.g., hydroxyl groups.
The substantially homogeneous blend may serve either as the base-soluble component of an unexposed resist, or as an acid-labile material (e.g., as a result of containing acid-cleavable pendent groups such as acid-cleavable esters) that releases acid following irradiation as a result of the photoacid generator in the resist composition.
Another embodiment of the invention provides a photoresist composition comprised of the substantially homogeneous polymer blend described above and a photoacid generator.
The present invention also relates to the use of the photoresist composition in a lithography method. The process involves: (a) coating a substrate (e.g., a ceramic, metal, or semiconductor substrate) with a film comprising a radiation-sensitive acid generator and a copolymer as provided herein; (b) exposing the film selectively to a predetermined pattern of radiation to form a latent image therein; and (c) developing the image using a suitable developer composition. The radiation may be ultraviolet, electron beam, or x-ray. Ultraviolet radiation is preferred, particularly deep ultraviolet radiation having a wavelength of less than about 250 nm (e.g., 157 nm, 193 nm, or 248 nm). The pattern from the resist structure may then be transferred to the underlying substrate. Typically, the transfer is achieved by reactive ion etching or by an alternative etching technique. Thus, the compositions of the invention and resulting resist structures can be used to create patterned material layer structures, such as metal wiring lines, holes for contacts or vias, insulation sections (e.g., damascene trenches for shallow trench isolation), trenches for capacitor structures, etc., as might be used in the design of integrated circuit devices.
Additionally, the invention relates to a method for improving the aqueous base development of a lithographic photoresist composition comprising a polymer transparent to deep ultraviolet radiation and a radiation-sensitive acid generator, wherein the improvement comprises incorporating into the lithographic photoresist composition an additional polymer comprised of a monomer unit having the structure of formula (I).
The invention also relates to a method for reducing the optical absorption in the vacuum ultraviolet region of a lithographic photoresist composition comprising a polymer transparent to deep ultraviolet radiation and a radiation-sensitive acid generator, wherein the improvement comprises incorporating into the lithographic photoresist composition an additional polymer comprised of a monomer unit having the structure of formula (I).
In another embodiment of the invention, a method is provided for preparing the preferred copolymer comprised of a first monomer unit having the structure of formula (II) and a second monomer unit having a structure selected from formulae (I), (III), and (IV). The method involves copolymerizing, via direct free radical polymerization (e.g., bulk free radical polymerization) in the presence of a free radical initiator, a first monomer having the structure of formula (V): 
wherein R3, R4a, R4b, and R5 are as described above and a second monomer having the structure 
wherein R1, R2, and L are as described above. Additional monomers such as those having the structure of formula (VII) or formula (VIII): 
wherein R1, R2, R6, R7, R8, and R9 are as defined above, may also be incorporated into the copolymer. Again, any of the unsubstituted carbon atoms shown in the above molecular structures may be substituted with one or more inert, nonhydrogen substituents as described earlier.